Interfering atoms could help detect gravitational waves

Scientists in California have proposed a new type of gravitational-wave detector that is immune to laser noise – a problem that adds to the expense of current detector designs. The researchers believe that their proposal – a modified form of an atom interferometer – would be cheaper and easier to implement in space than current laser interferometers.

Gravitational waves are tiny perturbations in the curvature of space–time that arise from accelerating masses – according to Einstein's general theory of relativity. The first hint that the waves exist was spotted in 1974 as a gradual decrease of the orbital period of the pulsar PSR B1913+16, which circles a neutron star. However, no-one has directly detected a gravitational wave. Such a discovery would provide confirmation of general relativity and also open a new field of gravitational-wave astronomy, in which distant objects could be studied by the waves they emit.

Huge and costly

The conventional way to try to detect gravitational waves involves a long-baseline laser interferometer. A passing gravitational wave should cause the pathlengths of the two beams to change slightly, causing a shift in the interference fringes when the beams are recombined. None of these detectors have yet succeeded in detecting a gravitational wave; so to increase sensitivity, astronomers need to put detectors in space. Constructing a traditional L-shaped interferometer in outer space would require three satellites, which poses severe technological and financial challenges. The proposed Laser Interferometer Space Antenna (LISA) project, originally scheduled for launch in 2015, has been revised because of its high cost.

A single-baseline interferometer, which measures the change in length of a single path by interfering the emitted and reflected waves in a mirror cavity, would require just two satellites. But in this set-up it would be difficult to distinguish changes in pathlength from random fluctuations in the frequency of the laser – a phenomenon called phase noise.

Atom interferometers were proposed in the late 1980s and first built in the early 1990s by physicists including Mark Kasevich and Steven Chu at Stanford University. Instead of measuring the difference in phase between two beams of light, an atom interferometer measures the change in the phase of a matter wave made of atoms in a superposition of quantum states. An atom interferometer can be created by repeatedly exciting and de-exciting one half of the wavefunction using a laser while holding the other half in the ground state. The wavelength of an atom shortens when the atom is in its excited state, creating a phase shift between the two halves of the wavefunction that depends on how long the first half has spent in the excited state.

Each atom cloud is like a stopwatch Mark Kasevich, Stanford University

In this latest work, Kasevich and colleagues, led by theoretical physicist Peter Graham at Stanford University, propose placing two atom interferometers a long distance apart and using the same pulsed lasers – one originating at one interferometer, one at the other – to excite and de-excite the atoms in both interferometers. The time each atom spends in the excited state depends on the travel time of the laser pulses between the two atom interferometers. "Each atom cloud is like a stopwatch," explains Kasevich. "When the laser pulse comes from one direction, it starts the clock. When it comes from the other direction, it stops it."

Putting atoms to work

If the length of the baseline between the interferometers is constant, the atoms at both interferometers will accumulate the same phase shift. But if one interferometer accelerates relative to the other, the time between excitation and de-excitation of half the wavefunction will differ at the two locations and the atoms will accumulate a different relative phase shift. The same laser pulses excite and de-excite the atoms in both interferometers, so the laser-phase noise affects both atoms in the same way and does not affect the difference between the phase shifts detected at the two interferometers. "The light is just acting as the gate to turn off and on the clock," says Kasevich. "The atom is doing all the hard work."

Gravitational-wave expert B S Sathyaprakash of Cardiff University is cautiously optimistic. "The scheme is obviously very exciting," he says. "But I think the big question is what kind of technology is required in space to run this thing for three to five years? I'm not saying anything negative or positive, but I would like to see numbers." In an attempt to provide these, the Stanford team is currently planning to build a prototype in the laboratory to ascertain whether or not there are any unforeseen technical challenges with the proposal.

No alv, it should be understood. If you're a flatlander in a rubber-sheet world, when a gravitational wave comes along and stretches your rubber sheet, don't expect to be able to measure it with your rubber ruler.

No alv, it should be understood. If you're a flatlander in a rubber-sheet world, when a gravitational wave comes along and stretches your rubber sheet, don't expect to be able to measure it with your rubber ruler.

Well, exactly. As Eddington pointed out already before many years, gravitational waves do not have a unique speed of propagation. The speed of the alleged waves is coordinate dependent. A different set of coordinates yields a different speed of propagation and such waves would propagate like the noise. Everyone of you can detect gravitational waves with TV set - they do manifest itself with CMBR noise.

Relativists use a simplified form of Eistein field equations to calculate various properties of his gravitational field, including Einstein gravitational waves, which are based on the Einstein's pseudo-tensor. This simplified form is called the linearised field equations. They do this because Einstein's field equations are highly non-linear (implicit actually) and impossible to solve analytically. So they use the linearised form, simply assuming that they can do so. However Hermann Weyl proved in 1944 already, that linearisation of the field equations implies the existence of a Einstein's pseudo-tensor that - except for the trivial case of being precisely zero - does not otherwise exist.

Where is this Eddington guy?

No alv, it should be understood. If you're a flatlander in a rubber-sheet world, when a gravitational wave comes along and stretches your rubber sheet, don't expect to be able to measure it with your rubber ruler.

Well, exactly. As Eddington pointed out already before many years, gravitational waves do not have a unique speed of propagation. The speed of the alleged waves is coordinate dependent. A different set of coordinates yields a different speed of propagation and such waves would propagate like the noise. Everyone of you can detect gravitational waves with TV set - they do manifest itself with CMBR noise.

Relativists use a simplified form of Eistein field equations to calculate various properties of his gravitational field, including Einstein gravitational waves, which are based on the Einstein's pseudo-tensor. This simplified form is called the linearised field equations. They do this because Einstein's field equations are highly non-linear (implicit actually) and impossible to solve analytically. So they use the linearised form, simply assuming that they can do so. However Hermann Weyl proved in 1944 already, that linearisation of the field equations implies the existence of a Einstein's pseudo-tensor that - except for the trivial case of being precisely zero - does not otherwise exist.

May be we should call him up and ask if he has seen those waves after leaving us ? Now, he may be in a better position to tell us all about it, unless he came back to re-learn?

The ultimate truth is always penultimately a falsehood. The commenter ALV, who knows , might well be right. We are in for big surprises when it concerns the physical reality of gravitation. During the last three centuries we have become more or less mesmerised and deeply convinced there is a force called gravity. True, as I have shown, there is an implication of potential energy correlated with GR and KE with SR. It is this PE which somehow,it is believed, gets converted into GW or GRad. This is what interferometric research is supposed to detect as gravitational waves. Now if gravity is not really a force, but the consequence of a force, then as ALV believes, there might not be any GRad or GW. Some scientists are perfectly entitled to keep an open mind on the issue or to have alternative views. That does not mean that General Relativity as a universal concept is threatened for it has proved its worth wonderfully well. The realities of the universe, it will turn out, will have quite a few surprises yet to come.

If it is not "Gravity", then what???

The ultimate truth is always penultimately a falsehood. The commenter ALV, who knows , might well be right. We are in for big surprises when it concerns the physical reality of gravitation. During the last three centuries we have become more or less mesmerised and deeply convinced there is a force called gravity. What we have been seeing is some sort of force doing what we call gravity would do, if it exists, which many think it does. Now the exact mechanism of gravity is a lot harder to understand, that is true. So if it is not gravity, then please explain what it is.